General David Petraeus announced today that he has resigned as head of the CIA, citing an extramarital affair.

The idea of a senior figure in Washington resigning because of infidelity seems quaint in the post-Clinton era of David Vitter and other still-active public servants who’ve put scandals behind them. Attention will quickly turn to Petraeus’ struggles to establish himself at the agency, the traditional ill will between the CIA and the uniformed military and, of course, Petraeus’ handling of the Benghazi attack.

But the real reason Petraeus was done in by something so easily shrugged off by others in Washington is much simpler than that. In the world of spies, there is a short list of weaknesses you can exploit in your opponents; they include ideology, greed, indebtedness and infidelity. People have spent centuries figuring out how to get people to commit treason. Arguing that the other side is better for humanity, offering to help someone who’s deeply in debt or paying hard cash to the greedy are classic methods. But few are more effective than blackmail, and even in the post-Lewinsky era in Washington, infidelity leaves officials vulnerable.

You can’t lead an organization whose case officers must be impervious to blackmail if you’re vulnerable to it yourself.

Here’s Petraeus’ statement:

HEADQUARTERS Central Intelligence Agency

9 November 2012
Yesterday afternoon, I went to the White House and asked the President to be allowed, for personal reasons, to resign from my position as D/CIA. After being married for over 37 years, I showed extremely poor judgment by engaging in an extramarital affair. Such behavior is unacceptable, both as a husband and as the leader of an organization such as ours. This afternoon, the President graciously accepted my resignation.

As I depart Langley, I want you to know that it has been the greatest of privileges to have served with you, the officers of our Nation’s Silent Service, a work force that is truly exceptional in every regard. Indeed, you did extraordinary work on a host of critical missions during my time as director, and I am deeply grateful to you for that.

Teddy Roosevelt once observed that life’s greatest gift is the opportunity to work hard at work worth doing. I will always treasure my opportunity to have done that with you and I will always regret the circumstances that brought that work with you to an end.
Thank you for your extraordinary service to our country, and best wishes for continued success in the important endeavors that lie ahead for our country and our Agency.